There have been employed distributed control systems in recent years that control distributed target devices to be controlled, such as sensors or actuators, using a network in order to save the wiring in industrial equipment or factory automation (FA).
Typically, a distributed control system includes a central communication device which centrally controls the entire sequence and multiple terminal communication devices which are connected to target devices to be controlled and control the input and output of the target devices to be controlled. In the distributed control system, the central communication device and terminal communication devices form a network. This network is typically constructed using a topology such as a multi-drop or daisy chain. The central communication device transfers control command information to the terminal communication devices through this network, and the terminal communication devices control the input and output of the target devices to be controlled in accordance with the control command information. The terminal communication devices also transfer input information from the target devices to be controlled to the central communication device through the network.
As seen above, in the distributed control system, the central communication device can centrally control all the target devices to be controlled on the network by transmitting or receiving control information, such as control command information or input information, to or from the terminal communication devices through the network.
In the field of industrial equipment, distributed control systems are required to provide control responses faster and have higher functionality as the speed and functionality of the equipment are increased. For this reason, the distributed control systems must transmit a great amount of control information at high speed and therefore are required to reduce the communication response time of the network and increase the amount of data which can be communicated.
These improvements in the performance of the network often require making changes to the hardware of the central communication device and terminal communication devices of the distributed control system or making changes to the software in accordance with changes in the communication control method. Designing these changes involves many man-hours, making it difficult to improve the performance of the distributed control system.
One conceivable method for solving these problems is to improve the performance of the network by changing the settings of the communication control method without changing the fundamental communication performance (electrical transmission speed, communication specification, or the like) of the distributed control system. That is, the man-hours are reduced by reducing the frequency with which a change is made to the hardware of the central communication device and terminal communication devices. Another conceivable method is to reduce the man-hours by automatically changing the settings of the network communication control method of the distributed control system.
Conventional technologies for implementing these methods include a technology disclosed in Patent Document 1. A system according to the Patent Document 1 classifies packets into at least two types, including high-priority communication packets and low-priority communication packets, on the basis of predetermined communication priority and inputs the classified packets, and then makes the respective types of packets redundant. This system employs a packet division method of, when transmitting low-priority packets through one packet communication network, dividing those packets in accordance with the target delay time and amount of communication data before making those packets redundant. This conventional technology prevents communications using high-priority communication packets from being blocked by communications using low-priority communication packets and therefore can be expected to improve the performance of the network by setting priorities to communication packets.